Microanalysis of inclusions in irradiated UO2

JOURNAL

OF NUCLEAR

MATERIALS

22

(1067)

MICROANALYSIS

33-40.

0

NORTH-HOLLAND

OF INCLUSIONS IN IRRADIATED B. M. JEFFERY

Central

Electricity

Board,

Generating

Berkeley Received

An

electron

examine

probe

microanalyser

inclusions

present

in

PUBLISHING

20 October

Berkeley,

Gloucestershire,

r&&s

highly

presents simultanbment

comme

&ant

les

principaux

tels que le rhodium,

le ruthdnium,

has been observed and usually found to be associated

cbrium, le nbodyme,

lo strontium

with the inclusions. Molybdenum

matrice

and/or barium have together with

other fission products

such as rhodium,

technetium,

neodymium,

zirconium.

cerium,

The UOz matrix

rut,henium,

strontium

d’UOz

Mit

stark bestrahltem

and neodymium.

Hilfe

einer Mikrosonde

insbesonderes & sonde Blectronique a 6th utilisi!

feste

sowie Rhodium,

d’uranium

dym,

soumis & une forte irradiation. La &g&gade fission solides a 6th observee et

clusions.

molybd$ne

1.

Le

qu’elle ct!ou

Zirkonium

le baryum

Lanthan,

ont

BtB

Commercial

Research

Spaltprodukte sind Molybdiin

Ruthenium, und

beobachtet. und/oder

Technetium,

Zirkonium.

Die

mit

don Elementen

Cer, Praseodym

Cer, NeoUOS-Matrix

der seltenen

und Neodym

Die

Barium

dass sie Erden

enthillt.

the reduction of UOZ at, high temperatures to UOzPZ which, on cooling below 1800” C, reverts to stoichiometric UOZ with an accompanying rejection of free uranium. Roake 5) and Bates 6) have reported white particles in zircaloy-clad UOZ irradiated with central temperatures high enough to cause extensive grain growth; these inclusions have been identified as p-uranium by room temperature X-ray diffraction analysis. Bleiberg et aZ.7) have reported the presence of white inclusions in irradiated UOZ as being a consequence of high exposure and high temperature of irradiation ; the zircaloy cladding could act as an oxygen sink but they rule out the identification of the inclusions as free uranium because of their resistance to chemical etching and suggest they could consist of fission product cations alloyed with uranium.

such phenomena as gas release and swelling have received considerable attention [for example the review by Childs I)] but information on the kinetics and relocation of solid fission products is more limited. Belle et aZ.2) have reported the occurrence of two types of white inclusions in irradiated UOz, one associated with iron in the original seed material and the other (the nature of which is uncertain) arising as an irradiation effect,. A similar white phase has been observed by Anderson 3) and Rothwell 4) in unirradiated UOz heated in oxygen-deficient atmospheres to address:

in

Es wurden

temperatures in excess of 1800” C. This has been identified as metallic uranium arising from

The progressive build-up of gaseous and solid fission products and the microstructural changes that may occur during the irradiation of uranium dioxide can affect its economic performance as a commercial nuclear fuel. The mechanisms of fission gas movement including

Present

Einschliisse

analysiert.

wurde ebenfalls geprtift,. Es wurde gefunden,

est associee aux in-

Introduction

t

Strontium

wurden

Urandioxid

Haupt,komponenten

pour examiner les inclusions pr&entes dans le bioxydc

en g&&al

et

has also been examined

corium, praseodymium

on a trouvb

praseodyme

n6odyme.

lanthanum,

tion des produits

et on a trouvb

terres rares tels quc lanthane, c&ium,

and

le La

du zirconium plus les Elements des

and found to contain zirconium plus the rare elements

Un microanalyseur

le technbtium,

et le zirconium.

a Bti? aussi examinee

qu’elle contenait

constituants

avec autres produit,s de fission

uranium dioxide. Segregation of solid fission products

been detected as the main constituents

UK

1966

has been used to irradiated

UOz

+ Laboratories,

Nuclear

CO., AMSTERDAM

Department, 33

Reckitt

and Sons, Ltd.,

Hull, IJK.

Itecently.

.Hradbwy

cf rlJ.8) Ita\~>examined

white inclusions found in high I)ruw1111samples of XT& which Ilad been irradiated in the Materials l’esting Reatrtjor. 1~‘I,I?I’O : using a microanalyser,

the solid fission ~)roducts molybruthenium. oerium aiid barium M-C’rc

denum. det,ectetl in lwalisetl ~ont:c,llt,ratiotis. Xlectron I)L‘O~CI microanalysis ha.s also been used by the present author for csamining a similar sample. provided be- A F:NSE Harwell. in ordw to iI~~~est,i~~t,~t,he o(~(‘~~r~*~i~(*~’ of solid fission produr:t,s in t,llew inclusions

and in the

IJOz mahris. 2.

Experimental

procedure

Several modifications wcrc made to an AEI microanalyaer tSo facilitate use with at:t,ivC samples : 1. a small dense metal t shield was fitted to the counter slit system to rednw the baakground count ; 2. the optical microscope was used with a right angle eyepicw t,o minimise any hazards when viewing the sample, the built-in shielding of the slw~cinicn chamber bc*ing least, around the microscope tjube ; 3. the sample holder w:as conrertetl to a push fit; for rapid handling purposes. since one of the limit,ing factors when using a microanalyser with gamma-active materials has been shown to be the handling of the active samples 0). If suitable cutSting and ~~olis~~ji~~facilities aw available t’hc specimen actjivit8y ~loed 11cvw exceed 50 mC’i and careful (ilroosing of Dhe n~i~~roa~~~~l~s(~r opwat.ing conditions would1 for most elemeut,s: l)ermitj sensitivit*ics to be WlIl~Ek rable wit’lr t81iosc obtaiwtl from non-aot,ivcb specimens. A diamond saw was used Taocut a t~ratlsvcrsc section, 1 mm thick, from a iliglIl~- enriched UOa pellet which had been clad in stainless steel and irradia~tetl t80 w 4.6 atI ‘j/i, burn-up. the estimatZed centre t,el~l~)erature during irradiation being 1500” C. A small fragment from this triwsverse section

MICROANALYSIS total fission yield. For microanalysis

OF INCLUSIONS purposes,

the fission products were considered in 2 groups : (1) elements having atomic number (2) where 37 ;Z<

45

A lithium

and (2) elements fluoride

crystal

with

55.~2~

62.

could be used to

analyse characteristic radiations from both groups, but the low peak to background ratios of the principal K emission lines from elements in group (1) together with the presence of many uranium

L lines in this region

spectrum

arising from excitation

the matrix gave low sensitivity.

of the X-ray of uranium in The minimum

detectable limit was improved however by analysing the L radiations from the group (1) elements using a mica crystal, the lower probe voltages necessary for such an analysis exciting only a few M lines from uranium, most of which were of low intensity. The lithium fluorido crystal was used for analysing the principal L lines from the elements in group (a), and a gas-flow proportional counter was used with both crystals throughout the analysis. The increased background count rate arising from the active nature of the specimen was further reduced, when necessary, using pulse height analysis ; for a spectrometer scan requiring pulse height analysis, the mean wavelength of the gate was kept proportional to the Bragg angle using calibration data.

4.

IN IRRADIATED

Experimental

4.1.

An

UOz

35

results

MICROANALYSIS OF THE INCLUSIONS example

of the microstructure

in this

fragment of the irradiated UOz pellet is shown in fig. 1. The inclusions, which appear rounded and white and usually

found

associated

with

grain boundary porosity. are small on average, being (5 pm in diameter. A total

of 26 inclusions

were

analysed

in

detail and several fission products were detected. Molybdenum was a major constituent of N !)O y0 of these inclusions

whilst N 30 o/o of the total

contained barium in large amounts ; every inclusion analysed appeared to be depleted in uranium with respect to the UOs matrix. The inclusions can be categorised into 3 main types containing molybdenum and/or barium as the main constituents : 4.1.1.

Precipitates containing without barium

molybdenum

This was the most common type of inclusion found, constituting N 70 o/o of the total number analysed. Rhodium, ruthenium, technetium and neodymium were invariably detected in the presence of molybdenum. Fig. 2 shows a typical X-ray spectrum emitted from this type of inclusion when an angular scan of the

Fig. 1. Micrograph of high burn-up UOS showing

white,

rounded

inclusions

( x 600).

l3.

31. JEFFERY

4.1.“.

Precipitates

containiny mol~ybdenum

A few of the inclusions

(-

with

15 o/“) contained

both molybdenum and barium as the main constituents. Rhodium, ruthenium and technetium

were

detected

of molybdenum)

(again in the presence

as shown by the spectrometer

scan of fig. 4. An unidentifiable found

at I)=-~13.70” whenever

elements

occurred

together.

peak (Q) was

these tM.0 main Cerium.

neodym-

ium, zirconium and strontium were also detected in this type

of inclusion

(figs. 5 and 6).

Precipitates containing molybdenum

4.1.3.

barium

witholct

v

I_

I Fig.

2.

I

6

I

emitt)ed

by

f3

14

I5 BRAGG

Spectromet,cr

I

1

ANGLE’

scan

of‘ t,hr! X-ray

a type

(I

spectrum

) inclusion.

The third group comprising the remaining 15 o/o of the inclusions analysed contained barium as the major fission product constituent together with cerium, neodymium, zirconium and strontium as in the other barium-containing inclusions of type (2). Rhodium, ruthenium and techneticum were not detected but as fig. ‘i shows, a very small amount of molybdenum

)

I

13

I

I2 BRAGG

r

I

ANG::

o

scan of a. type (1) inclusion Fig. 3. Spectrometer using pulse height analysis to show the presence of tellurium and pallatlium.

01

spectrometer was made ; using pulse height analysis it was also possible on several occasions to detect small amounts of tellurium and palladium, as shown in fig. 3.

I

I

1

16

I l3RAc’cs ANGL:’

Fig.

4.

Spectrometer

scan

showing

the presence

together denum.

of a type of barium

(2)

inclusion

and molyb-

MICROANALYSIS

OF

01 44

Fig.

5.

Spectrometer

43

scan

41

42

of

a

type

INCLUSIONS

40 eRaCGHhrJcLE”

(2)

inclusion

IN IRRADIATES

38

showing

37

UO2

I

37

the

presence

of

neodymium

and

cerium.

neodymium, lanthanum and praseodymium were homogenously distributed throughout the matrix but enrichments of zirconium and cerium occurred in barium-rich areas [i.e. in type (2) and (3) inclusions]. Typical measured concentrations, deduced from peak intensities obtained by slowly scanning the spectrometer through the requisite Bragg angle, are shown in table 3 ; the measured zirconium concentration was, however, determined with the spectro-

Fig.

6.

showing

Spectrometer

scan

the

of

presence

of

a type

zirconium

(2)

and

inclusion strontium.

was recorded. A trace of yttrium was detected in only one of these inclusions. Table 2 shows typical measured concentrations of elements detected in each type of inclusion. 4.2.

MICROANALYSIS

OF THE

UOZ

MATRIX

Using a static probe and pulse height analysis, the UOZ matrix was analysed in several places and zirconium, cerium, neodymium, lanthanum and praseodymium were nearly always detected.

Fig. 7. Spectrometer showing

barium with

sca,n of a type

as the

a small

major

trace

(3)

constituent

of molybdenum.

inclusion together

elcment~s of a.tfjacent, atomic

Element

Calculated

nwrrbc~r~.

(wt TO)

Mrosured

(w+Sj/$)

meter set on the zirconium Lix peak and point1 counts taken. This procedure. although giving greater accuracy of result. was rat’her t,imeconsuming and not used for the other elements.

whilst ruthenium, technetium, tellurium. rhodium and palladium will exist in the metallic state: the chemical state of molybdenum will depend on the oxidising conditions. From examination of phase diagrams, t,he rare earth oxides and zirconia should form solid solutions with UOs and this is clearly subs~.a~ltiate~lby the results of t#hematrix analysis. Apart from cerium, a,11the other detected rare earths i.e. neodymium, cerium. lanthanum and praseodymium. were found homogeneously distributed throughout t’he UOS matrix : yt~trium and sarnarillm were belo~v the de&&ion limit, of the method of analysis employed. The occurronce of areas enriched in cerium. which wer< generally associated with baril~m-rifle il~~Iusi(~ns, c*ould be due to segregation of a perc*ursor of one of the cerium isotopes formed duriug thermal neutron fission of 2WJ. The decay chains of the two sbable crrium isotopes. i@JCe and IWe, as reported by Iintcoff aw : a) 16s~~ laf)Xe -+ 66sec 3.8

5.

Discussion

The segregation of solid fission products can be best discussed by considering the Iikely chemical state of each isotope in uranium dioxide. Wait ii) has suggested from thermodynamic considerations, assuming thermal equilibrium conditions, that the rare earths, alkaline earths and zirconium will be stable as oxides

b) 1.6ser: ‘4”Xe 0.35

14%

--f

A

: 8sec *4%_?s--f 10mitl

HImin i%a where, for instance.

140Ba + 6‘..3 stable IaWe 6.4

l_‘.Stl

6.0 #t._‘h *doI,a i 6.3

*@ABa-_,-

--f stable i4sCe 6.0

16sec 1WXe --+ corresponds 3.8

MICROANALYSIS

OF

INCLUSIONS

IX

IRRADIATED

39

UO2

to a lQ-‘Xe isotope with a yield of 3.8 o/o decaying

stable in the metallic

with a half life of 16 set to a daughter product 141cs.

denum) and forming a solid solution. It is well known that although an electron probe diameter of c; 1 jrrn may be obtained for

Approximately cerium results includes

50 y. of the total yield of from a decay chain which

a 140Ba isotope

12.8 d. This would

reactor

with

provide

sufficient

for

Ba

temperature

microanalysis,

a half life of

the

yet owing to diffusion of electrons

of uranium in the inclusions could therefore be

to

segregate, a tendency that microanalysis has already shown occurs with the barium-rich

due to the electron

inclusions of types (2) and (3) before decaying via 140La to stable 14OCe.

the adjacent

inclusions

TABLE of measured

derived

from

product

Measured Type

of

Occurrence

precipit’ate

factor

ratios

occurrence

MO 29.0

0.7

3..5

12.5

9.0

0.15

0.4

1.4

0.9

3

0.15

Ratio

of measured

concentrations (Rh

taken

Ratio

3.9

13.9

9.9

3.6

2.5

3.6

2.0

as unity)

of fission

product

yields

taken

as unity)

from

talole 2

factor

Tc

1

1.0

u-it,h rat,ios

Ru

2

Total

(Rh

1

over an inclusion

small into

UO2 matrix causing excitation of experimental evidence However,

yields 1”)

concentration r

Rh

or spreading

penetrat’ing

4

concent,ration fission

beam

U X-rays. suggests that uranium is present in all the inclusions analysed, varying in concentration according to the type of inclusion ; greater amounts of uranium were detected in bariumrich inclusions, especially those in which molybdenum was absent. although this difference is probably magnified by the fact that the calculated diffusion depth in a molybdenumrich precipitate is N l-l.5 jtrn compared with N 3-3.5 /lrn in a barium-rich precipitate. Measured uranium values are probably accurate to only * 5 wt %. Table 4 summarises the extent to which the present microanalysis results can account for the relocation of several fission products into inclusions. By considering the major fission product constituents of each type of inclusion,

Although neodymium was detected in most inclusions, no enrichment with respect to the matrix was ever measured and results of analysis shown in table 3 indicate that this element is homogeneously distributed throughout the fuel, probably as Ndz03. Zirconium, as expected, was readily detected in the UOZ matrix. probably as Zr02, but was also measured enriched with respect to the matrix in barium-rich inclusions. Examination of the liquidus curves of alkaline earth oxides with zirconia could account for this occurrence of zirconium and also strontium with barium since there exists a strong tendency towards compound formation 12). The molybdenum-rich inclusions contained several transition elements, each probably

Comparison

molyb-

in the sample the zone of X-ray production is larger than the actual probe size. The presence

time at

isotope

state (including

Ba -

3.2

3.0

_

3.75

32.2

1 6.75

excluding

cerium and zirconium which were also

detected

in the matrix, it can bc seen t,hat the

experimentally determined ratios of t,he con centrations of rhodium : ruthenium : teehnetiurn: molybdenum: barium (l:Xfi:2.5:8.3:1.7) in good agreement with the theoretical ratios (1: 3.6 : 2.0 : 8. J : 1 .!t) determined from the fission

are

yields in table five elements, segregate

1. This would suggest that8 all unlike most of the rare earths.

completely

into

inclusions ; c&urn

Three kinds of inclusion were found, contain ing rnol~~)de~lurn and/or c*onstituents.

‘I’llC:

barimn

palladium were also det~eoted in small amounts. Bari~~m-ricll inclusions were always associated with enrichment~s of c+erium, zirconium and strontium.

Uranium

inclusion.

the smallest

was

detect,etl

experimental

homogeneous

6.

Conclusions

White rounded inclusions, observed in UOa irradiated to 4.6 at %, have been examined with an electron probe microanalyser. Some solid fission products were found segregated and were usually identifiable with the inclusions whilst others were detected in a llomogeneous distribution throughout, the UOg matrix.

The

matrix

U&

in

every

~~railiurn ~oi~~entratio~~

being in hhose free of barium.

analysis. In a preliminary examination of the tfannverse section of the irradiated UOa pellet, the specimen was broken during remote handling operations and the exact location of t’he small subsequently retrieved for microfragment, analysis, wit,hin the section was unknown. The effect of temperat8ure gradient on precipitate size and distributions was therefore not examined. ~xperimeIlta1 resulm did not however show any tendency for segregation of a particular type of inclusion to occur within the fragment, examined.

ill-

elusions invariably contained rhodium. ruthtnium. technetium and neodymium ; t~ellurium and

and zirconium, occurring in only Y 30 o/o of the inclusions analysed, can be accounted for within error by the results of the matrix

as the main

Inolybdeilum-bearing

was found

to

contain

distribution oflanthanum.

a

cerium,

praseodymium. neodymium and zirconium alt8hough enhanced ~?on~elltrat,i[)lls of cerium and zirconium were found associated with bariumrich imlusions. This paper is published by permission Central Electricity Generating Board.

References

of the